The present invention relates to oligomeric compounds with acid-labile protective groups, to processes for the preparation of these compounds and to positive-working radiation-sensitive mixtures containing these compounds as solubility inhibitors. It also relates to a recording material prepared with these compounds, which is suitable for photoresists, electrical components and printing plates and also for chemical milling.
In the industrial manufacture of microelectronic components, a number of lithographic techniques are currently used, different demands being made in each case on the photoresist mixtures used therein. Thus, in g-line lithography, where radiation of a wavelength of 436 nm is employed, conventional diazonaphthoquinone/novolak photoresists are generally used. A more recent development is i-line lithography, in which radiation of a wavelength of 365 nm is used. Images with details of a mask original in improved resolution down to 0.5 .mu.m can be obtained with this technique. More recent modifications, such as phase-shifting mask technology, permit a further reduction in image size down to about 0.35 .mu.m and even finer. Even better resolution will become possible in the future by means of the "UV-2" photoresists. A distinction is here made between two variants: UV-2 wide-band irradiation (240 to 260 nm) and irradiation with KrF-excimer lasers (248 nm).
As can already be seen from the above, the limit of resolution is given by the wavelength of the radiation used. The continuing reduction in size of the structural dimensions, far down into the submicron region, especially for microchips, requires modified lithographic techniques. Because of their short wavelength, high-energy UV radiation, electron beams or X-rays are here particularly suitable. A review of the demands that are made on the radiation-sensitive mixtures used in a particular case, will be found in the article by C. G. Willson, "Organic Resist Materials--Theory and Chemistry" (Introduction to Microlithography, Theory, Materials and Processing; Editors: L. F. Thompson, C. G. Willson, M. J. Bowden; ACS Symp. Ser., 219: 87 (1983), American Chemical Society, Washington). There is therefore an increased demand for radiation-sensitive mixtures that can be used in the advanced technologies, especially in mid-UV lithography, deep-UV lithography, electron lithography and X-ray lithography. In addition, they are preferably sensitive within a wide spectral range and can thus also be used in conventional UV lithography.
A frequently used positive-working radiation-sensitive mixture for producing radiation-sensitive recording materials contains an o-quinonediazide derivative and a binder that is soluble in aqueous-alkaline solutions, for example, a novolak or a polyhydroxystyrene. However, the sensitivity of the recording materials to UV radiation, in particular high-energy short-wave radiation, for example, to the light of a KrF-excimer laser having a wavelength of 248 nm, or to electron beams is generally insufficient.
Positive-working radiation-sensitive mixtures in which a photo initiator generates an acid as a result of the action of actinic radiation show an improved sensitivity. In a subsequent reaction, this acid cleaves an acid-cleavable material and thereby renders it soluble in aqueous-alkaline developers.
It is also known that compounds having phenolic OH groups can be "masked" by tert.-butoxycarbonyl groups. Acids cleave this derivative into the phenolic starting compound, carbon dioxide and isobutene. Such compounds can also be utilized as light-sensitive solubility inhibitors.
Radiation-sensitive mixtures with acid-cleavable solubility inhibitors always require a small quantity of a compound that, on irradiation, generates an acid that in turn effects the cleavage of the above-mentioned materials. As photolytic acid generators, onium salts, such as diazonium, phosphonium, sulfonium and iodonium salts of non-nucleophilic acids, such as HSbF.sub.6, HAsF.sub.6 or HPF.sub.6, have especially been used. In addition, halogen compounds, especially trichloromethyltriazine derivatives or trichloromethyloxadiazole derivatives, o-quinonediazidesulfochlorides, o-quinonediazide-4-sulfonic acid esters, organometal/organohalogen combinations, bis(sulfonyl)diazomethanes, sulfonylcarbonyl-diazomethanes or nitrobenzyl tosylates have been recommended.
Such mixtures, some of which have high sensitivities to actinic radiation, are called photocatalytic 3-component systems, since they contain, as essential constituents, a polymeric binder soluble in aqueous-alkaline solutions (in most cases a phenolic resin), a photoactive compound and an acid-cleavable solubility inhibitor. Among these mixtures, those have gained particular acceptance in practice which contain compounds with acetal groups as the acid-labile component, since these combine adequate cleavability on the one hand with adequate storage stability on the other hand, especially in the dissolved form. The acetal must here have, inter alia, a largely hydrophobic molecular backbone, in order to be able to function as a solubility inhibitor. Acetals having free phenolic hydroxyl groups are completely unsuitable as solubility inhibitors, since they enhance the solubility in aqueous-alkaline solutions.
Furthermore, it can be observed generally that the process window, i.e., the spectral range of transmission for the exposure of these mixtures, is very narrow and frequently not unambiguously reproducible, causing inaccurate reproductions of the original. The inadequately narrow process window manifests itself especially in a steep dependency of the quality of image reproduction on the time difference between exposure and development, the so-called delay time. The causes of this deterioration in the image reproduction are not known in detail or have not been adequately investigated. In principle, it must be assumed that diffusion processes, which cause this behavior, cannot readily be controlled. However, it may be supposed that, during the drying of the mixture on a substrate material, a partial vaporization of the photoinitiator or of the acid-labile compound or a segregation of the individual mixture constituents takes place. This is particularly frequently observed in the case of acid-labile compounds having a low solubility in the usual coating solvents.
The decisive disadvantage of the known compounds containing acetal groups is the fact that the solubility differentiation between exposed and unexposed image areas, resulting from the cleavage of these compounds, is generally insufficient. It appears that either the acetal derivative used as the solubility inhibitor has an inadequate inhibiting property and, in addition to the exposed image areas, those which have not been exposed are also severely attacked and worn off during imagewise differentiation, or that the exposed areas do not have an adequate solubility to allow imagewise differentiation during development. The problem is that the known compounds are unable to provide a material that causes a sufficiently large solubility difference between exposed and unexposed areas. Whereas this effect is still generally acceptable in the case of the novolak resins used according to the state of the art, it is observed, when other polymers are used, that the known acetal derivatives virtually cease to show any inhibiting action and therefore no longer allow an image differentiation as required in practice.
In the acid-catalyzed cleavage of 1 mol of the acetal, 1 mol of the corresponding aldehyde and 2 mol of alcohol are formed. In general, the alcohol contributes to the improved solubility in alkaline developers. By contrast, the aldehyde reduces the solubility, so that it is frequently evaporated out of the mixture by means of an additional baking step. It is more advantageous, however, to leave the aldehyde in the layer composed of the mixture, since it can be evaporated only in an uncontrollable manner, especially in the case of different layer thicknesses. This leads to non-reproducible results with respect to sensitivity and development behavior.
In both cases, best results are therefore not obtained. If the solubility-inhibiting aldehyde generated by cleavage has to be evaporated out of the mixture, an additional processing step is necessary in order to obtain the best possible solubility of the exposed areas. If the aldehyde having an inhibiting action remains in the mixture, a differentiation of the solubility between exposed and unexposed layer areas, as required in practice, is not achieved. This has in turn particularly disadvantageous consequences if the mixture is used in a recording material. In this case, longer exposure and development times must be accepted, and the resulting relief image has, due to the inadequate solubility differentiation between exposed and unexposed areas, weaknesses in contrast, in the structural profile and in the wearing-off in the dark.